In a conventional ship propulsion system, a three-phase AC current motor having a short-circuited rotor is used as the motor, the rotor being mounted on a hollow shaft, which is coupled via a coupling to the drive shaft running inside the hollow shaft. The drive shaft is directly coupled to the propeller. In this ship propulsion system, the stator of the motor is fit into a tubular frame, which in turn is inserted into a pipe-saddle-like housing part of the housing, secured, gondola-like, on the lower side of the ship hull by a shaft-like supporting part. The cooling of the motor, including the bearings of the rotor, is carried out using fresh water, which is pumped from a tank arranged in the ship hull into the interior of the gondola-like housing and is circulated. Such a conventional system is described in, for example, U.S. Pat. No. 2,714,866.
In another conventional ship propulsion system, which can be designed for propulsive outputs of 10 MW and more, the dynamoelectric motor having its stator is supported in the surrounding housing by radially arranged web plates. The web plates used for this purpose also function to form cooling ducts (pipes) for a gaseous coolant, supplied from the ship hull and re-cooled there using a heat exchanger. In this context, the costly cooling device is arranged on the shaft-like supporting part. Conventionally, a synchronous motor, which is supplied by a frequency converter and which has a separately excited rotor, is used as the motor, the rotor being able to be additionally cooled through being arranged on a hollow drive shaft that has water flowing through it. Such an arrangement is described in, for example, U.S. Pat. No. 5,403,216.
For the purpose of cooling an electric motor that is driven under water, it is also conventional to pump in circulation the insulating oil used as a coolant so that in the cooling ducts which run axially in the area of the housing wall, heat is dissipated into the ambient water. Such a conventional system is described in, for example, U.S. Pat. No. 2,862,122.
It is furthermore conventional for the coolant circulation of a driving engine (prime mover) of a ship arranged in the ship hull to use a heat exchanger that is integrated into the jacket ring of the propeller. Such circulation is described in, for example, British Patent No. 2 260 805.